As my dad is living out his last years, we moved him into a spare bedroom in my house. Given his age, sometimes he is unable to control his bodily functions. This bedroom has never been a high traffic area and was built with real 2.25 x .75 white oak wood flooring. The oak was natural (unstained) and still has its original Minwax oil-based polyurethane finish from 20 years ago. I have wanted to temporarily protect the floor with something like one solid piece of 6' x 9' vinyl flooring in the bed area while dad is there. But sometimes I have seen products like this can soften the polyurethane and stick to it, basically ruining the poly finish :( . Any material suggestions, possibly something under the vinyl, that would would protect the wood and finish, even after a couple of years ??

This is the only time I will ever recommend this, but I think it's the prefect situation for a floating laminate (pergo-type) flooring. It would be applied over that foam-polyethylene underlayment, and the glued joints will exclude any leakages. When the time comes to restore the room, the original floor underneath should be unaffected. It's only fastened down by the shoe molding, essentially. It is very cleanable and hygienic. My only concern is that it may be slippery, but perhaps there are different grades of slick finish available.
S_M

This is the only time I will ever recommend this, but I think it's the prefect situation for a floating laminate (pergo-type) flooring. It would be applied over that foam-polyethylene underlayment, and the glued joints will exclude any leakages. When the time comes to restore the room, the original floor underneath should be unaffected. It's only fastened down by the shoe molding, essentially. It is very cleanable and hygienic. My only concern is that it may be slippery, but perhaps there are different grades of slick finish available.
S_M

Thanks, I really appreciate the suggestion! I was actually playing with the idea of using that same foam-polyethylene underlayment and putting it under the vinyl sheet flooring. But what concerned me is that both the polyurethane finish and the foam-polyethylene are organic carbon based and that the out-gasing of the new polyethylene could still soften the polyurethane and cause it to stick. I know, this is the type of question even a chemist would have to test in a lab to know the result. ;)

Sounds like you're over thinking this.
If the finish on the floor is 20+ years old it has fully cured by now and is less likely to be affected by what your describing. However, you could just lay some rosin paper down then the vinyl over that.